When an immersible unit is lowered from a floating platform, by means of a cable deployed from a cable deployment unit, a depth selection and control system is necessary to stop the deployment of the cable, within an acceptable level of accuracy, at the desired underwater depth for the unit. The deployment of a transducer from a sonobuoy is an example of an application wherein an immersible unit must be deployed a certain distance beneath the surface of the water.
Depth selection in present cable deployment units is often limited to several distinct levels, as, for example, a shallow depth of 30 meters, a medium depth of 100 meters, and a deep depth of 300 meters. The selection of depths is fixed because the cable deployment unit can only be deployed fully, partially, or not at all, and can only be deployed partially by using a pretied hanger configuration which is formed permanently during the manufacturing process and which therefore cannot be adjusted to permit a partial deployment of other than constant value.
A further difficulty with present cable deployment units can arise during the lowering of the immersible unit. When the deployment of the cable is suddenly stopped, as, for example, at the fixed medium depth, the cable undergoes severe stress because of the momentum of the descending cable deployment unit. If the stress on the cable is not kept within an acceptable limit, the cable could break at that time.
Yet another difficulty often arises because components of the cable deployment unit can become tangled with the deploying cable, arresting the descent of the cable deployment unit. This is most likely to occur when the cable is displaced from the vertical or when the cable is subject to oscillatory motions.
A system for providing variable depth control for an immersible package has been disclosed in U.S. Pat. No. 4,143,349. The system disclosed therein uses a spring-biased braking arm on top of a coil pack, which is released to pivot against a center post (around which turns of the coil are payed out) in response to a signal generated in the immersible package at the required depth, thereby braking the descent of the immersible package. However, this system depends on the existence of continuous tension in the cable to maintain tightness around the post. In the ocean, wave motion can cause the cable tension to vary, and this can result in repeated cycles of loosening and retightening of the first several turns of cable around the post. This action can create repeated and severe concentrations of stress on the cable, which in turn can cause the cable to break because of fatigue stresses. In addition, the position of the post in the middle of the cable pack might cause the cable to become entangled during the deployment thereof.